Tim’s 2021 Dream about 1975 Amp Repair

Tim’s 2021 Memorial Day Dream About 1975 Amp Repair in Oregon City.

I vaguely (unreliable) remember telling part of the following story to tech support whiz, Jason Beyer at Watlow-Winona, about a decade ago, but now it relates even more to my day-job’s recent controller developments.

While drinking this morning’s coffee, I remembered a dream from about 15 minutes before, almost a personal history movie scene replay of my worst troubleshooting day of 1975, working at Brownell Sound and HiFi in stinky Oregon City, south of Portland on the Willamette River, not far from the paper mill. One afternoon a guy brought in a nice Yamaha guitar & keyboard amp with an unusual stiff white Styrofoam speaker. He said someone had set a full gin-and-tonic glass on his amp and then knocked it over, spilling the sweet and sour fluid all over and into the amp, killing its sound but not blowing the fuse. He needed it fixed immediately: he had another gig that night. I suspected that the drink was his but did not wish to embarrass him by asking.

All this part is true, as I remember it.

Hooking it up, I confirmed that the amp did not amplify its input signal — all it produced was a robust 60 cycle hum. So I took it apart and saw that the circuit board and power transistor/heatsink assembly were heavily splattered with a clear sticky fluid that had not entirely dried. I did not taste it with a wet fingertip, though I considered doing so; I accepted the owner’s story.

All the wire connections and the attachment hardware had to be removed before I could wash the two assemblies in a sink with warm water and a clean paint brush, followed by a rinse with an isopropyl alcohol solution. I had high hopes; this was going to be a bit time-consuming but technically easy. Happy days, those, with a new-born baby and a singing wife awaiting my return home to our rented rustic A-frame cabin with wood heat in the Mount Hood foothills.

I carefully dried the assemblies with towels and a hair drier, reinstalled and hooked them back up. Testing time. Good news – my guitar signal was amplified; bad news – way too much 60-cycle hum remained. Short story – the clock ticked toward closing time, and my ire rose as I probed the signal chain, searching for a possibly damaged component to replace. The oscilloscope display showed hum at every level from tens of microvolts to whole volts through the linearizing high-gain feedback loop that supported the featured low harmonic distortion spec. Suspecting an ultra-thin residue of the wicked drink clinging like a drowning person to everything in reach, I redoubled my cleaning routine: removing, washing, drying, and testing, not once, but three more times before that amp was gig-ready.

Now the dream took a shocking turn as my brain’s busy neural network rose up through shallower levels of sleep, ultimately jolting me fully awake and laughing. Final assembly and inspection of the Yamaha amp revealed a previously unnoticed Watlow RML temperature-limiting safety module mounted on a little DIN rail near the speaker! It had apparently escaped the bad splash from the spilled drink, or at least it wasn’t interfering with the sound. I didn’t have time to investigate; it was nearly five o’clock, and I still had to write up my repair invoice and give the amp to Mr. Brownell so he could call the guitar player and complete the transaction.

Now fully awake, I wondered, could a single conductive fluid event (say, coffee with cream and sugar), if spilled into the passive venting slots of an RML (hosting both temperature Control and safety Limit outputs) conceivably force them both into an unsafe power-output-enabled state … the comic element being the (at least) three-decade anachronism of conflating a temperature controller that wasn’t invented until after 2005 into a 1975 dream scene. A silly and happy way to greet the new day!

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